The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to ultra-fast scans in which all of the NMR data for an image is acquired in less than one minute.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence the magnitude of the net transverse magnetic moment M.sub.t depends primarily on the length of time and the magnitude of the applied excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited spin induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.t. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.31 t/T*.sub.2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of RF excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more RF excitation pulses (B.sub.1) of varying magnitude, duration, and direction. Such excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.t over a range of resonant frequencies. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
NMR data for constructing images can be collected using one of many available techniques. Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least an RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting NMR signal. As is well known, the NMR signal may be a free indication decay (FID) or a spin-echo signal.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. Whereas the more conventional pulse sequences have repetition times TR which are much greater than the spin-spin relaxation constant T.sub.2 so that the transverse magnetization has time to relax between the phase coherent excitation pulses in successive sequences, the fast pulse sequences have a repetition time TR which is less than T.sub.2 and which drives the transverse magnetization into a steady-state of equilibrium. Such techniques are referred to as steady-state free precession (SSFP) techniques. There are two well known SSFP pulse sequences used to produce images. The first is called gradient refocused acquired steady-state (GRASS) and it utilizes a readout gradient G.sub.x to shift the peak in the S+ signal that is produced after each RF excitation pulse toward the center of the pulse sequence. This pulse sequence is shown in FIG. 4. In two-dimensional imaging, a slice selection gradient pulse is produced by the gradient G.sub.z and is immediately refocused in the well-known manner. A phase encoding gradient pulse G.sub.y is produced shortly thereafter to position encode the acquired NMR data. To preserve the steady-state equilibrium, the effects of the phase encoding gradient pulse are nullified by a corresponding G.sub.y rewinder gradient pulse after the NMR signal has been acquired and before the next pulse sequence begins as described in U.S. Pat. No. 4,665,365. The phase encoding gradient pulse G.sub.y is stepped through a sequence of values during a scan to acquire NMR data throughout k-space. In a 128 view scan, for example, the phase encoding gradient is stepped in sequence from k.sub.y =-64 through k.sub.y =+64.
The second well known SSFP pulse sequence is called contrast enhanced fast imaging (CE-FAST) and it utilizes the S- signal that is produced just prior to each RF excitation pulse. In this pulse sequence the acquired NMR signal is an S- echo signal caused by the gradient refocusing of the transverse magnetization which would otherwise refocus at the next RF excitation pulse. For a detailed discussion of the CE-FAST pulse sequence, reference is made to an article by R. C. Hawkes and S. Patz entitled "Rapid Fourier Imaging Using Steady-State Free Precision", published in Magnetic Resonance in Medicine 4, pp. 9-23 (1987). It also includes a phase encoding pulse which is stepped sequentially through a set of views during the scan.
While scans performed with SSFP pulse sequences are much shorter than conventional scans, the contrast in the reconstructed images is lower due to the use of limited flip-angles and the acquisition of data under steady state conditions. More recently, ultra-short TR snapshot NMR imaging with higher image contrast has been described by A. E. Holsinger et al in abstract number 363 presented at the Eighth Annual Meeting of the Society of Magnetic Resonance Imaging held in February 1990 and entitled "Improved T1 Contrast In Ultra-Fast Scans." Snapshot imaging employs two phases: a contrast preparation phase and a data acquisition phase. The contrast preparation phase consists of a sequence of pulses which set up the desired contrast. For example, a selective 180.degree. RF excitation pulse followed by a wait time (T1) sets up T1-dependent longitudinal magnetization. The subsequent data acquisition phase consists of a series of very fast acquisition sequences such as GRASS, FLASH or CE-FAST, which operates on the prepared magnetization, gradually moving the spin system toward the steady state condition characteristic of SSFP pulse sequences. Prior to reaching this steady state condition, however, the image contrast is enhanced and resembles the contrast established by the preparation phase, which is comparable to conventional spin-echo and inversion recovery sequences.